During photography using a digital camera or a video camera for household use, recorded images are sometimes blurred due to movements, such as vibrations, of the zoom optical system during image capture that are caused by the hands of the operator of the camera shaking or by other factors. In known conventional techniques for preventing such blurring of images by optical compensation, specified lenses of the imaging optical system are moved perpendicular to the optical axis in order to provide a function of preventing blurring of an image that is photographed or otherwise picked up, as described for example in Japanese Laid-Open Patent Application Nos. H10-260355, H11-237550, and H10-260356. This function of preventing blurring of an image is also known simply as an image-blur prevention function.
Japanese Laid-Open Patent Application No. H10-260355 discloses a zoom optical system with an image-blur prevention function built into the zoom optical system. Specifically, this application describes a zoom optical system having a four-group construction that includes, arranged along the optical axis in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power. In this zoom optical system, the third lens group has two groups of lens elements having positive refractive power, and when movements, such as vibrations, occur that would otherwise blur a picked up image, optical compensation in order to prevent blurring is achieved by moving one of the two groups of lens elements within the third lens group perpendicular to the optical axis of the zoom optical system.
Japanese Laid-Open Patent Application No. H11-237550 also discloses a zoom optical system with an image-blur prevention function and having a four-group construction that includes, arranged along an optical axis in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power. In this zoom optical system, the third lens group has two groups of lens elements of negative and positive refractive power in order from the object side, and when movements, such as vibrations, occur that would otherwise blur an image, optical compensation in order to prevent blurring is achieved by moving a group of lens elements within the third lens group that is on the image side having positive refractive power in a direction that is perpendicular to the optical axis. Furthermore, it is constructed so that the following conditions are satisfied:8<f3/fw<250.3<|f32/f3|<0.75where                f3 is the focal length of the third lens group,        fw is the focal length of the zoom optical system at the wide-angle end, and        f32 is the focal length of the positive, image-side group of lens elements of the third lens group.        
The above conditions are satisfied in order to obtain a back focal length that is large enough and to improve the sensitivity of the zoom optical system to movements of the positive, image-side group of lens elements of the third lens group.
Additionally, Japanese Laid-Open Patent Application No. H10-260356 discloses a zoom optical system with an image-blur prevention function and having a four-group construction that includes, arranged along an optical axis in order from the object side: a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power. The third lens group includes a meniscus lens element of negative refractive power having an aspheric surface, and with its concave surface on the image side. In this zoom optical system, image blur that would normally occur in a picked up image, such as a photographed image, due to vibration of the zoom optical system is prevented by compensating movements of the third lens group in a direction perpendicular to the optical axis.
Recently, a zoom optical system has been desired that has a more compact construction overall and also has a more accurate image-blur prevention function. However, the zoom optical system of Japanese Laid-Open Patent Application No. H10-260355 discussed above provides only a small correction in the angle of view, and sometimes blurring of the image cannot be prevented. For example, the following condition is satisfied:1.3<fSL/f3<2.0where                fSL is the focal length of the group of lens elements in the third lens group that moves for image blur prevention, and        f3 is the focal length of the entire third lens group.        
However, because the refractive power of the group of lens elements in the third lens group that moves for image-blur prevention is small, this group of lens elements may have to move a large amount in order to prevent blurring of an image, and also the overall length of the zoom optical system becomes large. This tends to increase the overall size of the entire device. Also, in the zoom optical system of Japanese Laid-Open Patent Application No. H11-237550 discussed above, the following condition is satisfied:8<f3/fw<25where                f3 and fw are as defined above.        
However, the refractive power of the third lens group may be small, and the amount of movement of the positive, image-side group of lens elements of the third lens group may be excessive for sufficient image-blur prevention. Moreover, because the group of lens elements of the third lens group that are on the object side of the third lens group has negative refractive power as a whole, the back focal length of the zoom optical system tends to become long. Therefore, the entire construction of the device tends to become large. Furthermore, in the zoom optical system of Japanese Laid-Open Patent Application No. H10-260356 discussed above as well, because moving the entire third lens group for image-blur prevention increases the size of the mechanism for the movement, miniaturizing the entire construction becomes difficult.